The invention relates to a method for reprocessing of used lubricating oils by alkaline treatment of the oil or of its raffinate at elevated temperature as well as reprocessing by one or several of the following steps; i.e. acidification, distillation, hydrogenation, bleaching clay filtration.
According to known methods for reprocessing used lubricating oils by alkaline treatment, and the successive chemical and/or physical processing steps, the impurities present in a heterogenously dispersed or dissolved state are removed. The alkaline treatment has for its purpose, the separation of as large a part of the impurities present in the used oil as possible in advance, so that the chemical and/or physical methods of purification may be carried out as smoothly as possible and with a small expenditure in apparatus, and in a material-saving manner. Acidification, hydrogenation or distillation, filtration with bleaching clay or a combination of these measures come into consideration as chemical or physical types of processing.
It has been known to treat used oil with caustic chemicals in order to achieve thereby the separation of the impurities required for an effective purification. The alkaline treatment has above all the advantage over an acid treatment according to the sulfuric acid process in that the waste materials are less harmful to the environment than is the "acid tar" developed by the sulfuric acid process, and in that because of resides in the quantities of the ash contained in the used oil and of the chemicals added to the used oil.
A distillation is advantageous as a final purification step since the filtration will encounter difficulties without previous acidification or expensive washing out. The also possible hydrogenation is eliminated generally because of costs.
The reprocessing methods for used oil, described in the literature heretofore and operating with an alkaline treatment step, still possess an additional, essential disadvantage, which up to now formed an obstacle for its introduction into practice. In case of the known reprocessing methods with individual alkaline-reacting chemicals, such as caustic soda, caustic potash, soda, sodium silicate or calcium hydroxide or their aqueous solutions, only an unsatisfactory separation of the impurities was achieved. Moreover, partly very long processing times were needed, which are prohibitive especially for an economic operation of a continuously operating installation.
However, whenever one puts up with an insufficient preliminary purification, only either qualitatively low grade secondary raffinates were obtained, or else one was forced into increased expenditure in the final purification step for example, with a fractionated distillation. Since impurities in the used oil reduce the cracking temperatures of the oil fractions, one was forced in case of incomplete preliminary purification to distill at lower temperatures, for example, in case of most used oils at temperatures below 300.degree. C. Therefore, the distillations could only be carried out at a high vacuum in very expensive apparatus.
According to U.S. Pat. No. 3,625,881 the proposal was made, to precede the fractionated distillation of used oils by a simultaneous treatment with easily boiling hydrocarbons and an alkali metal hydroxide solution. This was to produce an easier distillability and qualitatively improved products. In this case, 0.2 to 2.0% by weight of an alkali metal hydroxide in at least 40% aqueous solution are added to the dried oil at temperatures between 93.degree. and 149.degree. C. However, it turned out in practice, that the described measures were not sufficient in order to bring about the separation of a sufficiently large part of the impurities and to achieve a lubrication oil, qualitatively of equal value as a primary raffinate. Only in case of used oils with a low degree of contamination does this method appear usable.
U.S. Pat. No. 3,304,255 describes a method for the continuous reprocessing of lubricating oil from diesel engines of ships during their operation. In the described method, a partial stream of the lubricating oil is washed with a diluted alkali metal hydroxide solution, as a result of which there is achieved an enlargement of the particles and an easier flocculation of the impurities. In detail, one operates with the addition of 0.6% sodium hydroxide in the form of, preferably, 10%, maximally 20% solution. The processing time amounts to about 15 minutes at a temperature below 100.degree. C. This process too, may not be applied successfully to the used oils obtained in practice, which are often further strongly contaminated with additional oily waste products.
According to the Japanese patent application 73-49.801, used oil may be pre-purified prior to hydrogenation by a simple treatment with 2 vol. % of a 20% aqueous NaOH at 25.degree.-80.degree. C. and settling times of about 36 hours. However, it turned out, that this type of preliminary purification is not sufficient for the subsequent final purification by distillation. Moreover, in case of the used oils customarily obtained, no useful phase-separation will be achieved, so that the execution of this process encounters difficulties in practice.
Therefore, it was necessary to search for a process which has the advantage of the alkaline reprocessing method, is applicable universally to the broad spectrum of used oils collected in practice and not only applicable to used lubricating oils with a low degree of contamination, and which at the same time leads to raffinates, which qualitatively are the equals to the original raffinates without a requirement for expensive apparatus which would prohibit the technical realization of such a process.